1. Field of the Invention
The present invention relates to a liquid crystal display circuit for use in an apparatus which is required to always display information such as a clock time, timer operating time, etc. even in a state where the main operating power supply such as a home AC power source does not operate. Such an apparatus is provided with a driving means which can operate by either the home AC power source or a battery, or an auxiliary power source such as a battery which serves to prevent the information such as the clock time from being lost by a failure of the home AC power source. Such an apparatus includes an audio device with a compact disk player called "CD radio-cassette", an audio device such as a radio with a clock, a video device such as a video cassette recorder with a clock, etc.
2. Description of the Related Art
In recent years, home electric apparatuses have been developed to have integrated functions to a very high degree. For example, the audio device called "CD radio-cassette" incorporates the functions of a clock, timer, radio, cassette tape recorder and compact disk player.
There is also an audio device such as a radio with a clock and a video device such as a video cassette recorder with a clock, in which their main functions are combined only with a clock and/or a timer.
Many products among the above devices, which have been on the market, are designed so that the same liquid crystal display device serves as both displays for a clock or a timer and for the other main function. These products are provided with a driving means which can operate by either a home AC power source or a battery, or an auxiliary power source such as a battery which serves to prevent information such as the clock time from being lost by a failure of the home AC power source. The purpose thereof is to always display, on a liquid crystal display device, information such as a clock time, timer operating time, etc. even in a state where a main operating power supply such as a home AC power source is not available.
FIG. 7 shows a structure of a part of the prior art liquid crystal display device related with the present invention. In FIG. 7, 1 denotes a liquid crystal display (LCD); 2 denotes a display driving means; 3 denotes a bias producing means; 5 denotes plural power supply means including a first power supply means 6 for supplying electric power from a home AC power source and a second power supply means 7 for supplying auxiliary electric power from a battery; and 8 denotes a power supply switching means.
FIG. 8 is a circuit diagram used to produce 1/3 biases in the bias producing means 3. The voltage V.sub.DD supplied from power switching means 8 is divided by R1, R2, R3 and R' to provide output voltages V.sub.LC0, V.sub.LC1 and V.sub.LC2 which are used as bias voltages in the display driving means 2.
In the prior art display circuit, normally, R1=R2=R3. Therefore, the relationship among the bias voltages can be represented by EQU V.sub.DD -V.sub.LC0 =V.sub.LC0 -V.sub.LC1 =V.sub.LC1 -V.sub.LCW =V.sub.LCD /3
where V.sub.LCD indicates an LCD driving voltage which can turn on the liquid crystal display. V.sub.LCD is selected to be a voltage substantially equal to V.sub.DD.
In this example, assuming R'=0 (.OMEGA.), the setting is made as shown below. EQU V.sub.LC2 =0(V), and V.sub.LCD =V.sub.DD
FIG. 9 is a waveform chart showing the driving voltages for LCD 1. LCD 1 displays information in a time-divisional manner by using two kinds of lines of "common (COM)" and "segment (SEG)". A certain display pixel at a position indicated by two lines of COM and SEG is turned on or off depending on whether or not a difference voltage between COM and SEG is an AC voltage exceeding a predetermined voltage (V.sub.LCD), at the timing when the display pixel is selected. Thus, assuming that the display data when a certain display pixel is to be turned on is "1", as shown in FIG. 9, the display pixel is turned on during a period when a large difference voltage of 2V.sub.LCD sufficient for turning-on is generated between COM and SEG at the timing of selection of COM and SEG, whereas it is not turned on in the other periods, i.e. when, even with the display data being "1", COM is not selected, and when the display data is "0", since, in these other periods, the difference voltage between COM and SEG is only 2/3 V.sub.LCD.
FIG. 10 shows an example of connection between LCD 1 and display driving means 2 when operating with 1/3 bias and 1/4 duty cycle to display a clock time, etc. Solid lines and dotted lines showing wirings within LCD 1 represent the connections of SEG lines and COM lines for applying signal voltages to the individual display pixels, respectively. In the example of FIG. 10, a timing control circuit in display driving means 2 produces selection signal voltages of COM 0 to COM 3 as shown in the timing chart of FIG. 11, which illustrates, as an example, signal voltage waveforms on SEG 2 line and respective COM lines and potential difference waveforms therebetween, and also signal voltages of SEG 0 to SEG 7 in accordance with the display data.
Thus, if the display data on the display driving means 2 shown in FIG. 10 are supplied to SEG's 0 to 7 in synchronism with the outputs from COM 0 to COM 3, the above-exemplified prior art display circuit can display 0.123, for example, on LCD 1 as seen from FIG. 10. Here, however, the display of 0.123 does not have a particular meaning such as a clock time.
As shown in FIG. 7, the voltage V.sub.DD to be supplied to bias producing means 3 changes in accordance with the state of power supply selected by power supply switching means 6 from plural power supply means 5 including first power supply means 6 for supplying electric power from a home AC power source and a second power supply means 7 for supplying auxiliary electric power from a battery.
Namely, first power supply means 6 maintains a stabilized voltage of e.g. +5 (V) or so, and becomes 0 (V) only when it is disconnected from the home AC power source, whereas second power supply means 7 changes its output voltage e.g. from +5 (V) to +3 (V) in accordance with the duration of use, because it includes a power source such as a battery.
The power supply is changed or switched from first power supply means 6 to second power supply means when the first power supply means 6 is disconnected from the home AC power supply or when a power source switching command signal A is applied by the operation of the device by a user.
However, the prior art LCD circuit described above has the following problems when it is put to use. Namely, as the voltage V.sub.DD to be supplied to bias producing means 3 changes, the display density (hereinafter referred to as "contrast") of display pixels on LCD 1 also greatly changes. Thus, the usable period of the second power supply means 7 for supplying auxiliary electric power from a battery, i.e. the life of the battery is reduced. Further, if the power supply is changed from first power supply means 6 to second power supply means 7 in the state where the battery voltage has been decreased, V.sub.DD will be abruptly decreased from +5 (V), thereby suddenly causing a great change in the contrast. In this case the user will receive an unusual impression of the display.